The preparation of cellulose ethers having uniform or different types of ether substituents is known [see, for example, Ullmanns Encyklopaedie der technischen Chemie ("Ullmann's Encyclopedia of Industrial Chemistry"), Volume 9, keyword "Celluloseather" (cellulose ethers), Verlag Chemie-Weinheim, 4th edition 1975, pages 192 et seq], these being prepared, in general, either (a) by the principle of Williamson's ether synthesis by reacting cellulose with alkyl or aralkyl halides (with the consumption of a base) and/or (b) or (c) by reacting cellulose with activated reactants (in the presence of catalytic quantities of a base): ##STR1## In these general equations:
______________________________________ Cell---O--H denotes, on the cellulose molecule, a hydroxyl group which is to be etherified, Hal denotes chlorine or bromine, R.sup.1 denotes an alkyl radical from C.sub.1 to C.sub.15, an aralkyl radical from C.sub.7 to C.sub.15, a carboxyalkyl radical from C.sub.1 to C.sub.3, a sulfonoalkyl radical from C.sub.1 to C.sub.3, a phosphonoalkyl radical from C.sub.1 to C.sub.3, a hydroxyalkyl radical from C.sub.1 to C.sub.6 or an N,N--dialkylaminoalkyl radical in which the alkyl groups, are from C.sub.1 to C.sub.3, R.sup.2 and R.sup.3 denote hydrogen or an alkyl radical from C.sub.1 to C.sub.13, R.sup.2 being identical with R.sup.3 or different therefrom, BOH denotes a base, such as NaOH or a quaternary ammonium base, and R.sup.4 denotes an optionally N--substituted carboxylic acid amide or sulfonic acid amide radical or a nitrile radical. ______________________________________
Mixed ethers cellulose are also prepared by the simultaneous or stagewise action of various etherifying agents on cellulose, in which connection, as well as a reaction process according to only one of the variants (a) to (c) indicated above, in particular, reactions according to at least two of the variants are also carried out. The following are examples of reaction products which are prepared by variant (a): methylcellulose (MC), benzylcellulose (BC), carboxymethylcellulose (CMC), sulfonoethylcellulose (SEC), phosphonomethylcellulose (PMC) or N,N-diethylaminoethylcellulose (DEAEC). The following are examples of reaction products which are prepared by variant (b): hydroxyethylcellulose (HEC) or hydroxypropylcellulose (HPC). The following are examples of reaction products which are prepared by variant (c): sulfonamidoethylcellulose (SAEC) or cyanoethylcellulose (CNEC). Mixed ethers of cellulose which are prepared by the same or different variant(s) of those indicated include, for example, methyl-hydroxyethylcellulose (MHEC), ethyl-hydroxyethylcellulose (EHEC), hydroxyethylhydroxypropylcellulose (HEHPC), methyl-carboxymethylcellulose (MCMC), hydroxyethyl-phosphonomethylcellulose (HEPMC) or methyl-hydroxyethyl-hydroxypropylcellulose (MHEHPC). Within the scope of subsequent statements, the term "cellulose ethers" is to be understood as meaning both products having a unitary substituent, such as hydroxyethylcellulose, and products having at least two different substituents, such as methylcarboxymethylcellulose.
Most known processes for the preparation of cellulose ethers are carried out in two main stages:
1. The preparation of the "alkali cellulose". PA1 2. The etherification of the cellulose molecule. PA1 They can undergo side-reactions with strong bases, for example diacetone alcohol is formed in this way from acetone. PA1 They can be at least partially immiscible with water, so that phase separation then takes place (for example in the case of alkanes, aromatic compounds or aliphatic ethers). PA1 Organic solvents containing hydroxyl groups or greater proportions of the latter contained in mixtures can compete with the polyhydroxy compound, cellulose, in reacting with the etherifying agents, so that the yield in the substitution reaction--relative to the cellulose--is reduced (for example in the case of alkanols or alkoxy-alkanols). PA1 They can be toxic, so that they are virtually excluded from modern commerical processes (for example, in the case of dimethyl sulfoxide or dioxane). PA1 The boiling point of the organic solvent is higher than that of water so that recovery by distillation--which is necessary in current processes--is unprofitable, and, in addition, the salts formed as by-products in the etherification then remain in the organic solvent [for example, in the case of ethylene glycol diethyl ether which has a boiling point (b.p. 760) of 123.5.degree. C.]. PA1 They can have a tendency to form dangerous by-products (for example, in the case of tetrahydrofuran the formation of peroxides).
In order to prepare the "alkali cellulose", cellulose in a finely-divided (for example ground) form is mixed as homogeneously as possible in suitable technical equipment with water and alkali-metal hydroxide (in general NaOH, but other bases, such as quaternary ammonium bases, are alternatively employed). The alkali-metal hydroxide is used in solid form or in the form of an aqueous solution. For the etherification reaction itself and thus for the quality of the end product of the reaction, the uniformity and intensity of the mixing is of decisive importance. The alkalization is generally effected at as low a temperature as possible, for example room temperature or below, in order to suppress degradation of the polymer (called "ripening"); however, under certain circumstances, for example the subsequent preparation of cellulose ethers of low viscosity, this degradation can also be desirable. The etherifying agent is optionally added as early as the alkalizing stage but, in this case, the temperature must generally be increased further in order to carry out the etherification reaction.
The actual etherification stage is generally carried out by heating the alkali cellulose, produced in the first stage, together with the etherifying agent (which has been added meanwhile), to temperatures between 30.degree. and 120.degree. C. Vigorous mixing in the second stage is also very important for the quality of the reaction product and for the cost efficiency of the process, since, for example, it is desirable to have a good yield in the substitution reaction while employing as small a quantity as possible of etherifying agent(s).
Both continuous and discontinuous procedures are known for both of the reaction stages. In the case of certain reactants, it is also possible to combine the two stages in such a way that pre-alkalization of the cellulose does not take place. Dispersing auxiliaries (suspending agents) are useful in both of the stages, or at least in one of the two stages, in order to achieve better mixing of the heterogeneous reaction mixture, and organic solvents which are either soluble in water or more or less insoluble in water are known from the state of the art.
They include, for example: ethylene glycol monoalkyl ether, ethylene glycol diethyl ether, dioxane, tetrahydrofuran, alkanols (in particular isopropanol or tert. butanol), alkoxyalkanols, toluene, heptane, mixtures of carbon tetrachloride and ethanol, acetone, methyl ethyl ketone; mixtures of benzene, toluene or xylene and ethanol; glycols, dioxane; mixtures of alkanes (of and above C.sub.6), aromatics, aliphatic ketones, aliphatic ethers or halogenated alkanes and alkanols (C.sub.2 to C.sub.4), dimethyl sulfoxide, dioxane or tetrahydrofuran; xylene, mixture of tert. butanol and acetone, mixtures of alkanes or aromatic compounds (C.sub.6 to C.sub.12) and alkanols (C.sub.1 to C.sub.4).
However, when used in the preparation of alkali cellulose and/or in the etherification of cellulose, the preceding organic solvents, which are known from the state of the art, exhibit at least one of the following disadvantages:
The prior, not prepublished, German Offenlegungsschrift No. 31 47 434 discloses a process for the preparation of cellulose ethers which is performed in the presence of water, bases and at least one inert, organic solvent, this solvent being dimethoxyethane. Particular attention is directed to this German Offenlegungsschrift with respect to reference to further prior art publications, process conditions, etc.